Add usage example for Celery

I recently wanted to collect custom application metrics from a rather large Celery app. I tried several approaches with varying degrees of awkwardness, until finally settling on having a "sidecar" HTTP server process for all Celery nodes.
This works pretty well, but took me a while to get right. To spare others from wasting as much time as me, I would like to propose adding a section on getting Celery metrics set up with the Prometheus client. I wrote up the solution on StackOverflow: https://stackoverflow.com/a/75799358/2532203

If you're interested, I can properly summarise it and open a PR?

Also, if you look at the code, I needed to copy some swaths of the library because the built-in Prometheus WSGI server is hard-coded to demonise the thread; this causes trouble when running in a demonised process. Maybe this could be made optional with another keyword argument that defaults to True?

Inizia con la soluzione di StackOverflow collegata nell’issue e con il riferimento al server WSGI del client Prometheus in exposition.py. Riassumi la configurazione del server HTTP sidecar per raccogliere le metriche dai worker Celery nella sezione appropriata della documentazione e verifica che l’esempio sia completo e comprensibile. Considera il comportamento opzionale di daemon-thread come una questione separata, a meno che la discussione esistente non la risolva.